Material Selection Guide

Selecting a Magnet

Stage 1 - Which Material - Choose the best material for your Application.

Stage 2 - What Shape - Choose a shape which suits your design.

Stage 3 - What Size - Use the web site to find the product code of the exact size or nearest we have in Stock.

Which Material?

Choice of magnetic materials is dependent on :

Magnetic Strength

Operating Temperature

Corrosion Resistance

Cost

Stability

For each of the categories above we will compare the four main material groups Ferrite, AlNiCo, Samarium Cobalt (SmCo), Neodymium Iron Boron ( NdFeB ).

The Energy stored in the material

Flux Density on the surface of the magnet

Pull required to remove the magnet from a flat piece of steel

Material

Energy ( MGOe )

Flux Density (Gauss)

Pull ( kg/cm2 )

Ferrite

3.3

1400

0.25

AlNiCo

5.0

1300

0.20

SmCo

26.0

3500

1.5

NdFeB

35.0

4500

2.5

Operating Temperature

All magnet materials lose magnetism as temperature rises :

Reversible Losses will return as the magnet cools as long as the working Temperatures are not exceeded. Exceeding working temperatures will mean that the magnet needs to be re-magnetised on cooling. Exceeding Curie temperatures will damage the magnet beyond repair.

Sub zero temperature affects are detailed below :

Material

Reversible Losses / Deg C

Working Temperature Deg C

Curie Temperature Deg C

Ferrite

0.19%

250

460

Large irreversible losses below -40 deg C

AlNiCo

0.02%

550

860

Losses < 10% down to 4 K

SmCo

0.03%

300

750

Minimal loses down to 4 K

NdFeB

0.12%

120

320

No losses down to 77 K

Corrosion

Material

Resistance to Corrosion

Additional Comments

Ferrite

Excellent

AlNiCo

Fair

SmCo

Excellent

Will break down if exposed to Hydrogen

NdFeB

Poor

Ni,Zn coatings protect in stores conditions. Will break down if exposed to Hydrogen

Cost

Material

Cost Comparison

Ferrite

Low ( x 1 )

AlNiCo

Medium ( x 5 )

SmCo

Very High ( x 20 )

NdFeB

High ( x 10 )

Stability

External magnetic fields

External magnetic fields can demagnetise materials to different degrees.

Material

Ease of Demagnetisation

Comments

Ferrite

Low

Pushing magnets together in repulsion can demagnetise by 10%

AlNiCo

High

Pushing magnets together in repulsion can demagnetise by 50%

SmCo

Very Low

NdFeB

Very Low

Time/Shock/Vibration

With modern materials magnetic losses with time and vibration are minimal. All magnet materials are brittle if subjected to impact, especially SmCo.

Radiation

It is recommended that magnets used in particle deflection systems have high Hci values. Tests have shown that SmCo shows significant losses when exposed to high levels of radiation ( 109 – 1010 rads ) and NdFeB also has high losses at 4 x 106 rads ( 50% ) and 7 x 107 rads ( 100 % ) Cobalt will also retain radiation after exposure.

Shape

For optimum performance it is recommended that you use the magnet materials in excess of the following Length to Diameter ratios. At ratios below this Gauss levels and pull could be reduced.